Writing instrument cap



FIG. 1 is a front elevational view of a writing instrument cap showing my new design;

FIG. 2 is a left side elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is an enlarged scale top plan view thereof; and

FIG. 5 is an enlarged scale bottom plan view thereof. 

The ornamental design for a writing instrument cap, as shown and described. 